1 Answer
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CONCAT() and & do the same thing, so in that case there isn't a difference between them (just personal preference I suppose)

CONCATENATE() can also be used to join 2 items, however it's typically used to join greater than 2 terms.

Which one is best? I'd say whichever option makes the formula the most readable, in this case I would say CONCAT() only because you can easily identify which function you're running, and the terms "TEST" and B2 are clearly defined as the arguments.